The invention is related to a signal converter for converting an input signal into an output signal, the signal converter including a combiner for deriving a combined signal from the input signal and a feedback signal and mapping arrangement configured for deriving from the combined signal both a plurality of samples of the output signal and the feedback signal.
Such a signal converter is known from U.S. Pat. No. 5,272,655.
Signal converters of this type are used, for example, in digital-to-analog converters operating according to the bit stream principle. In such converters, an input signal having a first sample frequency is transformed in an output signal having a second sample frequency. The input signal has a spectrum which is periodic with a period corresponding to the first sample frequency. Once the sample rate has been increased, the frequency spectrum of the output signal having the second sample rate continues to be periodic with a period corresponding to the first sample rate. However, a signal is desired which has a frequency spectrum that is only periodic with a period corresponding to the second (higher) sample rate. In order to realize this, a filter is required which eliminates the undesired frequency components. In a bitstream type converter, this filtering is performed by a low pass filter present in the feed forward path of the bitstream type converter.
A problem with bit stream type converters, especially with high ratios of the second and first sample frequency is the high operating frequency of the sample rate converter. In the signal converter according to the above mentioned U.S. patent, the operating frequency is reduced with a factor p. This is obtained by using a mapping arrangement which generates in response to one sample of the output signal of the combining means p samples of the output signal. The output samples are downsampled using a plurality of feedback filters. The feedback signals are combined with filtered input signals to obtain a plurality of difference signals. Each of these difference signals are filtered using a separate filter and finally combined into the combined signal. The number of filters is equal to the order of the filtering required, which can result in an increased complexity. This increased complexity leads to a larger amount of silicon area if the signal converter is realized in hardware.